(Relatively) Low rates of foreign immigration despite ageing population, strong central goverment with the intention of weakening external and internal influences (See Uyghurs / western chinese territories for an example of internal influences, for external influences the great firewall is an obvious one, but there are many others), and third a general ethno-nationalist mentality from many, but not all Han chinese, encouraged by the central state and the general culture.
I don't think those things necessarily answer the question. Also, how long has China been at extending their influence abroad? How long does these processes take? Obviously, in the case of Europe, this has been going on for centuries. Influence is inevitable because to influence others you have to be open to be influenced. Chinese that go to these countries to make deals, embassies with workers that grow culturally connected, this is the start. Relationships may tighten beyond this. I could go on.
That situation will be as untenable as Japan’s in the long run, as China also has its own looming demographic crisis. The government will likely open up to more immigration from Southeast Asia, at the very least.